Not owning a Canon 5DMk111 its impossible to comment on the “light leak” problem but having just spoken to Canon UK the understanding is that if you have a 5DMk111 with the serial numbers mentioned below Canon UK will repair the fault under warranty. NOTE : The updated firmware V 1.1.2 does not address this problem. (Note. Yesterday I had an email from Canon UK telling me that the new firmware addresses the light leak problem, sorry to all who read this genuine mistake).

FAQ: EOS 5D Mark III – Quality Issue Announcement

Issue

Concerning the EOS 5D Mark III digital SLR camera, when the LCD panel illuminates in extremely dark environments, the displayed exposure value may change. Canon has decided on our service policy to address this phenomenon, so we would like to inform you of the details as described below.

For example, “xxxxx1xxxxxx” or “xxxxx2xxxxxx” (x represents any optional number)

Cause

In extremely dark environments*, if the LCD panel illuminates, the displayed exposure value may change. However, when shooting with a lens attached, this change in exposure value will not significantly affect the shooting results.

* Some examples of a dark environment are as follows:

1. When the body cap is attached

2. When the lens cap is attached

Answer :

The products affected by this issue will be inspected and repaired free of charge after 10 May 2012 therefore you are kindly requested to contact one of our authorised service facilities.

We offer our sincere apologies to customers who have been inconvenienced by this. Canon strives to provide the highest quality products to our customers, and we spare no effort in our quality management to make sure our customers use our products with confidence. We hope our efforts will earn your understanding.

EOS 5D Mark III Firmware Version 1.1.2 [Mac OS X]

Description

Changes Firmware Version 1.1.2 incorporates the following improvements and fixes. 1. Supports a new accessory, GPS receiver GP-E2. 2. Fixes a phenomenon where a pink cast may develop over the image when the shutter is completely pressed with the camera’s power turned off (by the auto power off setting). 3. Fixes a phenomenon where the camera operation stops after one shot when shooting in High Dynamic Range (HDR) mode. 4. Fixes a phenomenon where the Shooting Date/Time in the EXIF data of the image shows a later time than the actual shooting time. 5. Fixes the time zone for the Samoa Islands. 6. Corrects errors in the Finnish menu screen. (Products for the Japanese market do not display the Finnish menu.)
Firmware Version 1.1.2 is for cameras with firmware up to version 1.0.7. If the camera’s firmware is already Version 1.1.2, it is not necessary to update the firmware.
Q&A Section: Preparations for the Firmware Update:
Macintosh After the downloaded compressed file (.dmg file) is extracted, a firmware folder is created.
*Extracting the downloaded file: The downloaded folder is automatically extracted, and a firmware folder is created. If the download folder cannot be automatically extracted, double-click the folder.
The extracted folder contains the firmware (file name: 5D300112.FIR / file size: 17,780,108 bytes) and instructions on the firmware update procedures (a PDF file in five languages: Japanese, English, French, Spanish, and Simplified Chinese).
Before starting the firmware update operations, please be sure to carefully confirm your understanding of these instructions.